Pin acts@main to pre-grid-regression commit to unblock build

[madbaron]

Pins acts@main to commit 0f9e9afe (parent of the breaking refactor) to unblock the nightly stack build.

Problem

The sim layer fails to compile k4actstracking against current acts@main (HEAD 739072bb):

Acts/Utilities/detail/MultiAxisHelper.hpp:192:70: error: subscripted value is neither array nor pointer
  192 |     multiIndex.at(N) = static_cast<std::size_t>(thisAxis.getBin(point[N]));

Instantiation chain:

CylindricalSpacePointGridCreator::fillGrid
  -> Grid<T,Axes>::atPosition(const Point&)            [Point = std::size_t]   # a global-bin index!
    -> Grid<T,Axes>::localBinsFromPosition(const Point&) [Point = std::size_t]
      -> MultiAxisHelper::getMultiIndexFromPoint -> point[N]                    # subscripts a scalar

Root cause

This is an upstream acts regression, not a k4ActsTracking/branch issue (k4actstracking main fails identically). In CylindricalSpacePointGrid.ipp, acts calls grid.atPosition(binIndex) with a std::size_t global-bin index; after the 2026-06-08 grid refactor (acts-project/acts#5541, c36cd88cd) that resolves to the position-lookup overload, so MultiAxisHelper subscripts a scalar.

• Bad: 739072bb (current acts main HEAD)
• Good: 0f9e9afe (parent of c36cd88cd, 2026-06-08) — pinned here

Verified the deleted buildcache forced a from-source rebuild that still failed, confirming the source (not a stale binary) is the problem.

Fix

Freeze acts@main to 0f9e9afe in environments/mucoll-common/packages.yaml. Revert to plain @main once acts main is fixed upstream (an issue is being filed at acts-project/acts).

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